"That would be an enormous, enormous factor in what's going on now", he added.
The president could try to appeal to the U.S. Supreme Court.
Trump has argued to in that case that he is immune from being investigated as a sitting president. The U.S. Supreme Court may eventually weigh in. Along with possible appeals in this case, a federal appeals court in NY is weighing a similar request for records from Trump's bankers at Capital One Financial Corp. and Deutsche Bank AG.
Slices of such information have since been made public by the media. They include documents from 2011 to 2018 that the House wants for investigation into the president's reporting of his finances and potential conflicts of interest.
On Friday, those arguments seemed to resonate with the U.S. Court of Appeals for the District of Columbia Circuit.
Democrats said they needed the records to understand whether laws pertaining to government officials and financial disclosures need to be changed.
'The most important question is not whether Congress has put forth some legitimate legislative objective, but rather whether Congress is investigating suspicions of criminality or allegations that the President violated a law, ' Rao wrote, declaring that the House of Representatives 'may not use the legislative power to circumvent the protections and accountability that accompany the impeachment power'. The firm has provided accounting services to Trump.
The House Committee on Oversight and Reform subpoenaed records from Mazars in April. A ruling in the president's favor in this case would have been "a very significant change in the balance of power, because it would represent a significant narrowing of congressional subpoena power", said David Sklansky, a law professor at Stanford University. District Judge Amit Mehta upheld the subpoena in May, and the Trump administration appealed. Trump immediately appealed. Oral arguments were scheduled for 23 October.
The president is also embroiled in lawsuits with the House Ways and Means Committee, which is trying to get six years of Trump tax records from the U.S. Internal Revenue Service and has the standing right to request similar information from the state of NY.
That other subpoena was sought by the Manhattan District Attorney's Office as part of an ongoing criminal investigation into how the Trump Organization accounted for hush money payments paid before the 2016 presidential election to two women, porn star Stormy Daniels and Playboy model Karen McDougal, in exchange for their keeping quiet about their alleged affairs with Trump years earlier.